Friday, March 14, 2014

Muscles work by getting shorter. We say that they contract, and the process is called contraction.

Muscles are attached to bones by strong tendons. When a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint.

Muscles can only pull and cannot push. This would be a problem if a joint was controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. The problem is solved by having muscles in pairs, called antagonistic muscles.

Biceps and triceps

The elbow joint lets our forearm move up or down. It is controlled by two muscles, the biceps on the front of the upper arm, and the triceps on the back of the upper arm. The biceps and the triceps are antagonistic muscles.

when the biceps muscle contracts, the forearm moves up

when the triceps muscle contracts, the forearm moves down.

This solves the problem. To lift the forearm, the biceps contracts and the triceps relaxes. To lower the forearm again, the triceps contracts and the biceps relax.

Muscles in the arm

Note to Students: Revise all the work you have done, there will be a TEST on Monday. BE PREPARED!

Wednesday, March 12, 2014

Basic structure

If two bones just moved against each other, they would eventually wear away. This can happen in people who have a disease called arthritis. To stop this happening, the ends of the bones in a joint are covered with a tough, smooth substance called cartilage. This is kept slippery by synovial fluid. Tough ligaments join the two bones in the joint and stop it falling apart.

The diagram shows the main features of a joint.

A Joint

Movement

Different types of joint allow different types of movement.

Hinge joints allow simple movement, the same as a door opening and closing. Knee and elbow joints are hinge joints.

Ball and socket joints allow movement in more directions. Hip and shoulder joints are ball and socket joints.

The bones cannot move on their own - they need muscles for this to happen.

Monday, March 10, 2014

Why do we need a skeleton?

Our skeleton is made of more than 200 bones. Calcium and other minerals make the bone strong but slightly flexible. Bone is a living tissue with a blood supply. It is constantly being dissolved and laid down, and it can repair itself if a bone is broken. Exercise and a balanced diet are important for a healthy skeleton.

The skeleton has three main functions:

to support the body

to protect some of the vital organs of the body

to help the body move.

Support

The skeleton supports the body. For example, without a backbone we would not be able to stay upright.

Protection

Here are some examples of what the skeleton protects:

the skull protects the brain

the ribcage protects the heart and lungs

the backbone protects the spinal cord.

Human Skeleton

Movement

Some bones in the skeleton are joined rigidly together and cannot move against each other. Bones in the skull are joined like this. Other bones are joined to each other by flexible joints. Muscles are needed to move bones attached by joints

Friday, March 7, 2014

We need to get oxygen from the air into the blood, and we need to remove waste carbon dioxide from the blood into the air. Moving gases like this is called gas exchange. The alveoli are adapted to make gas exchange in lungs happen easily and efficiently.

Here are some features of the alveoli that allow this:

they give the lungs a really big surface area

they have moist, thin walls (just one cell thick)

they have a lot of tiny blood vessels called capillaries.

The gases move by diffusion from where they have a high concentration to where they have a low concentration:

Oxygen diffuses from the air in the alveoli into the blood.

Carbon dioxide diffuses from the blood into the air in the alveoli

water vapour is also lost from the surface of the alveoli into the lungs - we can see this condensing when we breathe out on cold days.

Wednesday, March 5, 2014

The respiratory system

The human respiratory system contains the organs that allow us to get the oxygen we need and to remove the waste carbon dioxide we don't need. It contains these parts:

lungs

tubes leading from the lungs to the mouth and nose

various structures in the chest that allow air to move in and out of the lungs.

Ventilation

Movements of the ribs, rib muscles and diaphragm allow air into and out of the lungs. Take care - this is called breathing or ventilation, not respiration. When we breathe in, we inhale. When we breathe out, we exhale.

Parts of the respiratory system

Air passes between the lungs and the outside of the body through thewindpipe, called the trachea. The trachea divides into two bronchi, with one bronchus for each lung.

Each bronchus divides further in the lungs into smaller tubes called bronchioles. At the end of each bronchiole, there is a group of tiny air sacs. These air sacs have bulges called alveoli to increase their surface area.

Monday, March 3, 2014

Respiration

Respiration is a chemical reaction that happens in all living cells. It is the way that energy is released from glucose, for our cells to use to keep us functioning.

Remember that respiration is not the same as breathing (which is properly called ventilation).

Aerobic respiration

The glucose and oxygen react together in the cells to produce carbon dioxideand water. The reaction is called aerobic respiration because oxygen from the air is needed for it to work.

Here is the word equation for aerobic respiration:

glucose + oxygen → carbon dioxide + water (+ energy)

(Energy is released in the reaction. We show it in brackets in the equation because energy is not a substance.)

Now we will look at how glucose and oxygen get to the cells so that respiration can take place and how we get rid of the carbon dioxide.

Glucose from food to cells

Glucose is a type of carbohydrate, obtained through digestion of the food we eat. Digestion breaks food down into small molecules. These can be absorbed across the wall of the small intestine into the bloodstream.

Glucose is carried round the body dissolved in blood plasma, the pale yellow liquid part of our blood. The dissolved glucose can diffuse into the cells of the body from the capillaries. Once in the cell glucose can be used in respiration.

Oxygen from the air to cells

When we breathe in oxygen enters the small air sacs, called alveoli, in the lungs. Oxygen diffuses from there into the bloodstream.

Oxygen is not carried in the plasma, but is carried by the red blood cells. These contain a red substance called haemoglobin, which joins onto oxygen and carries it around the body in the blood, then lets it go when necessary. Like glucose, oxygen can diffuse into cells from the capillaries.

Red blood cells carry oxygen around the body

Carbon dioxide from cells to the air

The carbon dioxide produced during respiration diffuses out of the cells and into the blood plasma. The blood carries it to the lungs. It then diffuses across the walls of the alveoli and into the air, ready to be exhaled.